A series of novel trisulfonated poly(phthalazinone ether phosphine oxide)s (tsPPEPO) were synthesized using direct polycondensation. The properties of the achieved membranes including water uptake, swelling ratio, oxidative resistance, conductivity, and microstructure were studied in detail. The results demonstrated that tsPPEPO membranes with a high sulfonation degree showed an extremely low swelling ratio and outstanding oxidation stability without sacrificing proton conductivity, which is ascribed to the strong intermolecular interaction of hydrogen bonds and the particular morphology originated from the special distribution model of the -SO3H in tsBFPPO. Typically, tsPPEPO-120 exhibited a conductivity of 0.066 S/cm, a low swelling ratio of 11.5% at 80 degrees C, and almost no weight loss after being oxidized in Fenton's reagent at 80 degrees C for 1 h. The prepared membranes are expected to be alternative membranes for application in fuel cells.